lecture 3 - gene expression

-the double stranded DNA opens up -the RNA polymerase which comes through and it needs to locate on the gene a specific region so that it can begin transcription -the RNA polymerase moves along, you get your pre-mRNA transcript, that pre-mRNA transcript needs to be processed and in doing so it allows stability of that RNA transcript to remain within the nucleus so we can actually transcribe it -once the pre-mRNA has been processed and your exons have been removed, this mature mRNA transcript has a 5’ GTP cap and a poly(A) tail is protecting that transcript and that’s important as it allows for the stability of the RNA so it can be transcribed -RNA is very unstable it normally only is within the cell for anything from 5-20mins, it depends on the RNA transcript, so when we move that mRNA transcript out through the nuclear pores its targeted towards the ribosome, which is part of your RER -it’s within the ribosome that we translate that mRNA to make the protein

nucleus

nucleus

cytoplasm

amino acids

• a carbon with an amine group, carboxyl group, a hydrogen and a R group which varies depending on which amino acid we are looking at -has a simple structure

peptide bonds

-condensation reaction, removes a water molecule -the amine group of one amino acid and the carboxyl group of another amino acid join together

AUG

UAA,UAG, UGA

-degenerate -universal -written in a triplet code -non-overlapping -non-ambiguity - while the same amino acid can be coded by more than one codon, the same codon shall not code for two or more different amino acids -polarity - read in a 5’ to 3’ direction

-small -nucleophilic -aromatic -amide -hydrophobic -basic -acidic

-glycine -alanine

-serine -Thr -Cys

-Phe -Tyr -Trp

-Gln -Asn

-His -Lys -Arg

-Asp -Glu

-Val -Leu -Ile -Met -Pro

-the body makes the non-essential amino acids -a group of essential amino acids, these are the ones we need in our diet

-every region has 6 potential reading frames -your coding region is where the transcript is coming from -we read 3 bases at a time - a codon -you have the RNA transcript which will end at the end of the gene -if DNA is double stranded, you have antiparallel to that another strand which could be transcribed so there are more possible frames -how the cell decides what happens depends on the orientation of the DNA - you read from a 5’ to 3’ direction so on the other strand you would be reading in the other direction -you need a methionine and also one of the stop codons for transcription to occur -the one which is open will give the longest polypeptide

The first step of splicing involves U1 and U2. U1 binds to the 5 splice site and U2 binds to the invariant site or the branch point. In the second step the remaining snRNPs bind to the earlier bound snRNPs. U5 and U4-U6 bind to the intron region and hence, now the spliceosome is assembled. Now, the spliceosome loops out the intron, and the two ends of the introns are brought close to each other. Further U1 and U4 are released and U6 is bound to both, the 5 splice site and U2. In the next step the 5 end of the intron is cleaved and it attaches to the branch point of the intron, which is rich in A. Finally, the 3 end of the introns is also cleaved and the intron is released and further degraded by enzymes. The intron structure is called a Lariat (loop-like). Lastly the two exons are joined together. The snRNPs are used for splicing of other introns and the process of splicing continues for the remaining introns in the pre-mRNA.